Spectroscopic study of NO adsorption on magnesium oxide, nickel oxide and their solid solutions

The adsorption of NO on pure MgO at 77 K gives N2O2 (cis) dimers on {100} faces and NO2- (nitrite), N2O22- (hyponitrite) and N2O2 (cis) dimers on edges and steps. The hyponitrite anions (which are the precursors of N2O gas formation) and the dimeric N2O2 (cis) species are absent at room temperature, while the NO-2 species are formed in larger proportions. NO adsorption on dilute NiO-MgO solid solutions not only gives the species found on pure MgO but also surface Ni2+-NO nitrosyls; moreover, the presence of Ni2+ ions on the surface depresses the formation of N2O22- (hyponitrite) anions and favours the formation of N2O in the gas phase. On pure NiO, surface Ni2+-NO nitrosyls are the main product of NO adsorption at both room temperature and 77 K. The measured coverage is, however, only half the theoretical one, because repulsive forces prevent the occupation of all surface Ni2+ sites. The stability of the Ni2+-NO adducts prevents the extensive formation of N2O2 dimers.